Cognitive effort is emerging as increasingly important construct in a diverse set of literature ranging from predictors of everyday outcomes (e.g. academic achievement), to psychopathology, to fundamental principles of behavioral control. Effort may be particularly important, for example, for understanding disorders of anergia and amotivation. It may explain why individuals with major depression demonstrate intact performance on low effort tasks, yet perform below average on demanding tasks. Yet, despite rapidly growing interest, very little is understood about cognitive effort. Limited understanding stems from imprecise definitions and impoverished methodological frameworks. We propose to address this gap by using a novel approach, that adapting well established behavioral and neuroeconomic methods to formalize the notion of cognitive effort, in terms of a discounting index (i.e., the degree to which cognitive effort reduces the subjective value of task engagement). This cognitive effort discounting index provides an important leverage point from which to investigate individual differences in cognitive motivation, examine their brain basis, and identify the key mechanisms by which they contribute to decision-making regarding task engagement. The current project explores these issues through an integrated series of studies that involve a powerful within-subjects experimental design, sophisticated analytic methods, and cutting-edge fMRI methodology. Success in this effort will significantly advance our understanding of the basic neural mechanisms that contribute to decisions about cognitive effort, and will have important clinical implications by providing targets for diagnosis and intervention in disorders, such as major depression, for which cognitive motivational impairments play a major role.